Phosphorus is an important component of many commercial materials, including fertilizers, detergents, nutrients for animal feeds, corrosion inhibiting chemicals and others. The principal source of phosphorus is natural deposits of calcium phosphate. The calcium phosphate deposits are most commonly converted to industrial useful form by the digestion of such deposits, or phosphate rock, with sulfuric acid. Such digestion converts the calcium phosphate to phosphoric acid and various calcium sulfates, and is generally referred to as the "wet process phosphoric acid" production.
As is described in more detail below, the digestion of phosphate rock takes place at elevated temperatures in an aqueous slurry. Such slurry contains a high concentration of calcium sulfate solids together with other solids present in the phosphate rock. The potential recoverable phosphoric acid is within the liquid phase of such slurry. The separation of such solids from the phosphoric .acid-containing liquid phase is a critical step of the process
Such solids/liquid separation is accomplished by a filtration process. The filtration process is a highly significant factor in product yield and manufacturing costs. The efficiency of filtration process is dependent upon many factors, including the nature of the solids, the temperature at which filtration is conducted, the concentration of the solids in the material being filtered, the time restrictions, the nature of the liquid phase, and the filter media. Referring to such factors collectively as the "filtration conditions", the filtration conditions met in the wet process phosphoric acid production can be considered extreme.
Thus in wet process phosphoric acid production there is encountered both an extreme need for filtration efficiency and extreme filtration conditions. The filtration is normally the bottle-neck in phosphoric acid production.
Filtration aids, particularly polymeric filtration aids, are often used in industry to enchance filtration processes. The use of polymeric filtration aids in the wet process phosphoric acid production is also well known. The enhancement generally sought from filtration aids is the increase in filtration rate, the minimization or elimination of solids in the filtrate, and the minimization of liquid in the filter cake (retained solids).
In wet process phosphoric acid production the product itself is within the liquid phase, and it is extremely desirable to recover such product in concentrated form. Hence for the wet process phosphoric acid production both enhancing the filtration process and reducing the amount of acid associated with the filter cake (gypsum by-product) are very important.
Polymers are well suited for tailoring to specific needs. Polymers can be varied by their molecular weights, and the type and density of ionic charges or polar groups. Polymers may be nonionic, anionic, cationic or amphoteric. Nonionic polymers, without any ionic charges, may be varied drastically in nature by the type and density of polar groups, usually pendant from the polymer backbone, or even by the presence of significant nonpolar groups, or combinations thereof. Polymers with ionic groups can also contain pendant polar and nonpolar groups which can effect the overall polymer characteristics. Further, the environment in which the polymer is used can effect its characteristics, for instance the suppression of ionic charges in acid or alkaline media rendering the potential ionic groups essentially nonionic in nature.
When used as filtration aids polymers generally must be soluble in the liquid phase, usually water, at the concentration level(s) employed, and hence multiple types of polymer/liquid phase interactions are encountered. Multiple polymer/solids interactions are also encountered, and certain of such interactions may be predominant and/or be sought. When the predominant polymer/solids interaction is the reduction of surface charges on the solids, the polymer is commonly referred to as a coagulant, and is usually cationic in nature. When the predominant polymer/solids interaction is gathering together solids, or floc particles, by bridging from one surface to another, without any substantial surface charge modification, the polymer is commonly referred to as a flocculant, and is usually anionic or nonionic in nature. The role played by a given polymer is not defined, however, solely by its ionic or nonionic nature, particularly in complex solids/liquid separation systems, and other factors, including molecular weight of the polymer, the nature and concentration of the solids, and the nature of the liquid phase may all or in part determine the ultimate interactions and results thereof. Moreover, in dynamic situations, time itself is a highly significant condition that can control, in part, the extent of polymer interactions with both the liquid phase and solids, and hence the impact of the addition of the polymer to the system.
The present invention provides an enhancement to the filtration process, or step, of the wet process phosphoric acid production. Such enhancement concerns not only the generally desired goals of filtration enhancement, but also those particularly desired in wet process phosphoric acid production. The present invention provides a filtration enhancement under the extreme filtration conditions encountered in the wet process phosphoric acid production. The present invention employs polymeric filtration aids which provide the higher degrees of filtration efficiencies that are extremely desired in this important commercial production, while reducing the dilution usually associated with polymer introduction.